Vinyl chloride resin is excellent in its quality balance, as it is inexpensive and has good mechanical properties and chemical properties. In addition, vinyl chloride resin may form hard through soft molded articles when a plasticizer is added thereto. Therefore, vinyl chloride resin is a type of thermoplastic resin widely utilized in various fields. Its applications cover many fields, and vinyl chloride resin is therefore desired to have various properties necessary for such applications. For improving the necessary properties, not only vinyl chloride homopolymer resins but also vinyl chloride copolymer resins have been under investigation. For example, for improving the gelling property of plastisol prepared by dispersing a resin in a plasticizer so as to make it fluid, there is known a process of preparing a copolymer of a vinyl chloride type monomer and a vinyl monomer capable of forming a vinyl polymer having a low glass transition temperature (JP-A-63-23947). However, the process has some problems in that the both monomers may form homopolymers as polymerization reaction speed thereof often differs, and scales in the polymerization reactor increases.